The purpose of this study is to determine the effect of fluid restriction and the neurohormonal mechanisms in the development of hyponatremia in patients with congestive heart failure and hyponatremia. The hypothesis is that strict fluid restriction leads to a larger increase in plasma sodium than standard treatment in patients with decompensated heart failure associated with hyponatremia. A secondary hypothesis is that the neurohormonal change is greater in patients treated with strict fluid restriction versus standard treatment.